1. Field of the Invention
The present invention relates to a process for growing a crystalline compound semiconductor. Compound semiconductors are used, for example, as materials for composing various electronic devices, and for a miniaturization and improvement of the performance of electronic devices, sometimes it is desirable to grow a compound semiconductor having a required composition to a required thickness at a required place. The atomic layer epitaxy (ALE) process for controlling the growth at an atomic layer level is one means of attaining the above requirements.
2. Description of the Related Art
Known methods of a gas phase deposition of a crystalline compound semiconductor include a metal-organic chemical vapor deposition (MOCVD), a molecular beam epitaxy (MBE), and an atomic layer epitaxy (ALE), etc. The MOCVD provides a high deposition rate, but in a MOCVD, it is difficult to control the atomic layer level. The MBE uses a super high vacuum apparatus, wherein a molecule beam is fed into a super high vacuum chamber to grow a crystal layer.
ALE is advantageous when a ternary element compound semiconductor is grown. In the conventional processes, the three elements occupy random sites in the crystal so that scattering of a carrier is caused by the alloy effect. In contrast the site of each element can be designed by ALE so that a crystal structure without the alloy scattering effect can be grown.
Japanese Unexamined Patent Publication (Kokai) No. 61-34922 discloses an ALE in which a vacuum chamber is evacuated to a super high vaccum, as substrate is heated, and gases containing constituent elements for a compound semiconductor to be grown are sequentially introduced in predetermined amounts into the vaccum chamber, to grow a compound molecular layer by molecular layer. This process, however, requires a long time for switching the source gas, during which the once-deposited atomic layer may be adversely affected, and thus the controllability thereof is low.
Also, the ALE has problems with hetero epitaxy.
First, an epitaxial growth of an InAs layer on an InAs substrate is described. In a reaction tube of quartz or the like, an InAs substrate is heated to, for example, 350.degree. C., and a source gas for a III-group element, In, and a source gas for a V-group element, As, are alternately introduced over the substrate. The gas pressures are, for example, in a range of several torr to several 100 torr. An example of the In source is trimethylindium (CH.sub.3)In, and an example of the Ga source is arsine AsH.sub.3. An In layer and an As layer are alternately grown on the substrate, to thereby grow an InAs crystal by ALE.
Next, a GaAs crystal is grown, for example, at 500.degree. C., from trimethylgallium (CH.sub.3)Ga, as a Ga source and arsine AsH.sub.3 as an As source.
In the above examples, the growth temperature of an InAs crystal is 350.degree. C. and that of a GaAs crystal is 500.degree. C. Accordingly, when a heterojunction of InAs/GaAs is grown, if the growth temperature is set to 500.degree. C. it is too high for the InAs growth, and accordingly, the self-limiting effect is lost and an atomic layer growth becomes difficult. Further, if the growth temperature is set to 350.degree. C. it is too low for the GaAs growth, and thus crystal growth does not proceed. If the growth temperature is frequently varied during the crystal growth, the controllability and efficiency thereof are deteriorated.
Moreover, problems arise such as the differences of lattice constants and thermal expansion coefficients of crystals constituting the heterojunction, the stability of the respective atoms at the heterojunction, and an interdiffusion of constituent atoms at the heterojunction, or the like.
Furthermore, in a conventional ALE, the growth rate by one cycle of source gas supply is determined by the concentrations and supply times of the source gases. Particularly, the growth rate in ALE is reported, for example, for GaAs in Applied Physics Letters, vol. 53, pp. 1509-1511 (1988). Also, the purity of a GaAs crystal depending on the concentrations and supply times of the III and V source gases is reported in Journal of Crystal Growth, vol. 93, p. 557 (1988). Thus, the effects of the time when a source gas is not supplied on the growth rate and the characteristics of the grown crystal are not known.
As above, the atomic layer epitaxy, particularly hetero-epitaxy of a compound semiconductor has not been clarified as yet, and it is still difficult to grow a crystal having required qualities by ALE.